Always keep in mind, there is "normally" about a 20 degree (Fahrenheit) split between the vent temp and the temperature of the evaporator core. This is assuming, of course, the system is charged properly, not leaking and working as advertised.
If your vent temps are at 40 degrees, that means your evaporator temperature is around 20 degrees. Well below freezing, way too cold and "no bueno". More than likely, with that scenario, your evaporator is freezing up. Naturally, there are many variables to AC systems and proper charging. Never a "one size fits all" situation. Many more variables than can be addressed in a simple forum "quick reply". As always, YMMV.